Interleukin 15 fusion protein for tumor target therapy

ABSTRACT

Disclosed is a tumor-targeting fusion protein comprising at least (i) a IL-15 peptide or a variant or functional fragment thereof, (ii) a IL-15Rα polypeptide or a variant or functional fragment thereof, (iii) a Fc domain or a variant or functional fragment thereof, and (iv) a RGD polypeptide or a variant thereof. The fusion protein is preferably configured as RGD polypeptide-Fc domain-IL-15 polypeptide-IL-15Rα polypeptide. The tumor-targeting fusion proteins provided herein improves the anti-tumor effects of IL-15 and prolongs the half-life of IL-15, while targeting tumor sites and acting upon tumor cells. In addition, the fusion proteins are capable of being expressed at high efficiency and purified. The high efficiency of anti-tumor activity enables the fusion proteins to be an excellent candidate for tumor immunotherapy.

FIELD OF THE INVENTION

The present invention relates to the use of interleukin-15 in tumortargeting therapy, and in particular, to the anti-tumor activity of aninterleukin-15 fusion protein.

BACKGROUND

Cytokines play important roles in the regulation of the immune system,including anti-tumor immune responses. A number of cytokines have beenshown to have anti-tumor potential. Among these cytokines,interleukin-15 (IL-15) has been extensively studied as a promisinganti-tumor candidate. IL-15 belongs to the common receptor γ-chaincytokine family that also includes IL-2. IL-15 and IL-12 share the γ-and γ-chains of their receptors (IL-2/15βγ), but bind to different areceptor chains (IL-2Rα/IL-15Rα). IL-15 binds to IL-15Rα expressed onantigen presenting cells and the IL-15/IL-15 Rα complex then binds tothe IL-15βγ complex expressed on nearby effector cells. Similar toIL-12, IL-15 can stimulate the proliferation of T cells and naturalkiller (NK) cells, the expansion of cytotoxic T cells and the activationof NK cells. Unlike IL-12, IL-15 is not involved in theactivation-induced cell death and maintenance of regulatory T cells,which can block the therapeutic effects of IL-2. Thus, IL-15 is rankedat the top of the National Cancer Institute's list of agents with greatpotential for cancer immunotherapy.

However, recent studies have suggested that in vivo anti-tumor effectsof IL-15 could only be achieved at high dosage. Another limitation ofIL-15 as a therapeutic agent is its short plasma half-life. Onepotential limitation of these efforts is that the function of IL-15 issystemic and not tumor specific. In response to a long IL-15 half-life,cytotoxic T cells or NK cells are expanded systemically, not only intumors. As uncontrolled systemic activation of the immune system can bevery toxic and lethal, a more desirable therapeutic agent will need tobe able to limit its function to tumors and spare other tissues toreduce toxicity. Therefore, an ideal candidate should be tumor-targetingwithout substantial affects to normal tissues.

SUMMARY

The present invention provides a tumor-targeting fusion protein which,in one aspect, has improved IL-15 anti-tumor activity, and in anotheraspect, overcomes the problems associated with short half-life of IL-15.The fusion protein can target tumor site and act on tumor cells only.

The fusion protein provided by the invention comprises at least (i) aIL-15 polypeptide or a variant or a functional fragment thereof, (ii) aIL-15Rα polypeptide or a variant or a functional fragment thereof, (iii)a Fc domain or a variant or a functional fragment thereof, and (iv) aRGD polypeptide or a variant thereof.

In one embodiment, the fusion protein has components arranged asRGD-Fc-IL-15-IL-15Rα.

In one embodiment, the Fc domain is comprised of CH2 and CH3 of humanIgG1 and has an amino acid sequence SEQ ID NO:1 or an equivalent thereofthat has substitution, deletion or addition of one or more amino acidsbut retains the function of the Fc domain.

In one embodiment, the IL-15Rα is comprised of a IL-15Rα sushi domainincluding the subsequent 12 amino acidsfrom exon 3 and has an amino acidsequence SEQ ID NO:2 or an equivalent thereof that has substitution,deletion or addition of one or more amino acids but retains the functionof the IL-15Rα domain.

In one embodiment, the IL-15 has an amino acid sequence SEQ ID NO:3 oran equivalent thereof that has substitution, deletion or addition of oneor more amino acids but retains the function of the IL-15 domain.

In one embodiment, the RGD polypeptide has an amino acid sequence SEQ IDNO:4 or an equivalent thereof that has substitution, deletion oraddition of one or more amino acids but retains the function of the RGDpolypeptide.

In a preferable embodiment, the tumor-targeting fusion protein has anamino acid sequence selected from a group consisting of: (a) an aminoacid sequence of SEQ ID NO: 5; (b) an amino acid sequence encoded by anucleic acid sequence of SEQ ID NO:6; (c) an amino acid sequence encodedby a degenerate sequence of the nucleic acid sequence of SEQ ID NO:6;and (d) an equivalent amino acid sequence of SEQ ID NO: 5 that hassubstitution, deletion or addition of one or more amino acids butretains the function of the fusion protein.

In another aspect, the present invention provides a pharmaceuticalcomposition comprising a tumor-targeting fusion protein of the presentinvention and a pharmaceutically acceptable excipient including acarrier, a stabilizing agent and/or a vehicle. In one embodiment, thepresent invention provides a pharmaceutical composition comprising atumor-targeting fusion protein of the present invention and a furtheranti-tumor agent.

In one embodiment, the tumor is an integrin positive tumor, and inparticular, a αVβ3 integrin positive tumor including melanoma or ovariancancer. In one embodiment, the tumor is a progressive tumor, an advancedtumor, a tumor with a high burden/load, or a metastatic tumor.

In another aspect, the present invention provides a nucleic acidsequence encoding the tumor-targeting fusion protein, an expressionvector comprising the nucleic acid sequence, or a host transformed ortransfected by the expression vector.

In another aspect, the present invention further provides a kitcomprising a tumor-targeting fusion protein of the present invention, anucleic acid sequence encoding the tumor-targeting fusion protein, anexpression vector comprising the nucleic acid sequence, or a hosttransformed or transfected by the expression vector.

The tumor-targeting fusion proteins provided by the invention improvesthe anti-tumor effects of IL-15 and prolongs the half-life of IL-15,while targeting tumor sites and acting upon tumor cells. In addition,the fusion proteins are capable of being expressed at high efficiencyand purified. The high efficiency of anti-tumor activity enables thefusion proteins to be an excellent candidate for tumor immunotherapy.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a tumor-targeting fusion protein accordingto one embodiment (PFC-1). SP, signal peptide; RGD,Arginine-Glycine-Aspartate peptide motif; Fc, CH2 and CH3 of human IgG1;IL-15 Rα, IL-15Rα sushi domain+the subsequent 12 amino acids from exon3; L1, SS; L2, G₄S; L4, SG₂SG₄SG₃SG₄SLQ.

FIG. 2 is Coomassie blue staining of the fusion protein developed from10% SDS-PAGE under non-reducing (NR) or reducing (R) conditions.

FIG. 3 shows Mole proliferation stimulated by rhIL-15 and PFC-1. Theconcentration was calculated according to the molecular weight of aPFC-1 monomer. The data are shown as the mean±SD of triplicate samples.The results are representative of at least 3 experiments.

FIG. 4 shows CTLL-2 proliferation stimulated by rhIL-15 and PFC-1. Theconcentration was calculated according to the molecular weight of aPFC-1 monomer. The data are shown as the mean±SD of triplicate samples.The results are representative of at least 3 experiments.

FIG. 5 shows the PBMC proliferation stimulation in vitro by PFC-1.CFSE-labeled PBMCs were incubated with various concentrations of rhIL-15or PFC-1 for 6 d. Proliferation of PBMCs was assessed by flow cytometry.(A) Representative FACS images of PBMC proliferation are shown. (B)Quantitative analysis of PBMC proliferation stimulation by rhIL-15 orPFC-1. The concentration was calculated according to the molecularweight of a PFC-1 monomer. The data are shown as the mean±standarddeviation of triplicate samples. The results are representative of 3experiments.

FIG. 6 shows the binding of the fusion protein PFC-1 to HUVEC cell linesby flow cytometry. Results represented at least three independentexperiments.

FIG. 7 shows the binding of the fusion protein PFC-1 to SKOV-3 tumorcell lines by flow cytometry. Results represented at least threeindependent experiments.

FIG. 8 shows the binding of the fusion protein PFC-1 to LS74T tumor celllines by flow cytometry. Results represented at least three independentexperiments.

FIG. 9 shows the colocalization of the fusion protein PFC-1 withanti-human CD51/61 (αVβ3 integrin) antibody on HUVEC cell and SKOV3tumor cell models by laser confocal microscopy.

FIG. 10 shows the in vivo anti-tumor activity of the fusion protein inmouse. The mice were subcutaneously inoculated with B16F10 mousemelanoma cells on the back. When the size of the tumor reached 100 mm³in volume, the mice were intraperitoneally injected with 5 or 20 μgPFC-1, or 200 μl PBS every 3 days. The mice received two injections andthe tumor size was measured. Results represented at least threeindependent experiments (n=5-8 for each group). T-test was used forstatistical analysis, ** represents p<0.01.

FIG. 11 shows the in vivo anti-tumor activity of the fusion protein inmouse. The mice were subcutaneously inoculated with B16F10 mousemelanoma cells on the back. When the size of the tumor reached 1000 mm³in volume, the mice were injected via tail vein with 10 μg PFC-1, or 200μl PBS on scheduled dates. The mice received three injections and thetumor size was measured every day. Results represented at least threeindependent experiments (n=5-8 for each group). T-test was used forstatistical analysis, ** represents p<0.01.

FIG. 12 shows the phenotype change of CD8+ T cells of the mice receivedPFC-1 treatment in FIG. 10 by flow cytometry. Results represented atleast three independent experiments (n=5-8 for each group). T-test wasused for statistical analysis, ** represents p<0.01, *** representsp<0.005.

FIG. 13 shows the phenotype change of NK cells of the mice receivedPFC-1 treatment in FIG. 10 by flow cytometry. Results represented atleast three independent experiments (n=5-8 for each group). T-test wasused for statistical analysis, ** represents p<0.01, *** representsp<0.005.

FIG. 14 shows the phenotype change of CD44 antigen on the surfaces ofCD8+ T cells and NK cells of the mice received PFC-1 treatment in FIG.10 by flow cytometry. Results represented at least three independentexperiments (n=5-8 for each group). T-test was used for statisticalanalysis, ** represents p<0.01, *** represents p<0.005.

FIG. 15 shows the prevention of malignant migration of the fusionprotein PFC-1 in C57BL/6 mouse. On day 0, C57BL/6 mice were injectedwith 5*10⁵ B16F10 mouse melanoma cells via tail vein and received 10 μgPFC-1 or 200 μl PBS on the same day via tail vein. On day 21, the micewere euthanased and lungs were removed for assessments of the severityand number of pulmonary malignant tumor under binocular microscope. Thetop shows the lung of a presentative mouse. Mock, not received tumorcell injection; Vehicle, control received tumor cell injection; PFC-1,received PFC-1 injection. Results were expressed as mean±SD (n=5 foreach group). T-test was used for statistical analysis.

DETAILED DESCRIPTION OF THE INVENTION Fusion Proteins

As used exchangeable herein, the terms “fusion protein”, “PFC-1”, “PFC-1recombinant fusion protein” and “fusion molecule” refer to abiologically active polypeptide formed by more than one protein orpeptide sequences covalently linked (i.e., fused) by recombinant,chemical or other proper methods. A fusion protein can be fused to otherpeptide or protein sequence at one or more site through a linkersequence. Alternatively, a linker sequence can be used to assistconstruction of a fusion molecule. A fusion protein may exist in theform of a monomer or a multimer, e.g., a dimer.

In the present invention, a “fusion protein” comprises at least (i) aIL-15 polypeptide or a variant or functional fragment thereof, (ii) aIL-15Rα polypeptide or a variant or functional fragment thereof, (iii) aFc domain or a variant or functional fragment thereof, and (iv) a RGDpolypeptide or a variant thereof. In the fusion protein, components (i)and (ii) jointly constitute an effector module or molecule which caninduce the activation of effector cells (cytotoxic T cells and NKcells). Component (iii) is included to prolong the circulating half-lifeof IL-15. Component (iv) is a targeting molecule which acts with highaffinity and specificity on the receptor molecules expressed on thesurfaces of tumor cells, such that the remaining components are enrichedwithin the tumor site and kill the tumor cells.

In the invention described herein, the components of the fusion proteinare properly arranged such that the fusion protein achieves the expectedpurpose of the invention. In one embodiment, the components of thefusion protein are arranged as RGD polypeptide-Fc domain-IL15polypeptide-IL15Rα polypeptide. In another embodiment, the components ofthe fusion protein are arranged as RGD polypeptide-Fc domain-IL15Rαpolypeptide-IL15 polypeptide. In another embodiment, the components ofthe fusion protein are arranged as RGD polypeptide-IL15Rαpolypeptide-IL15 polypeptide-Fc domain. In another embodiment, thecomponents of the fusion protein are arranged as RGD polypeptide-IL15polypeptide-IL15Rα polypeptide-Fc domain. A person skilled in the artcan obtain the fusion proteins as described above by gene engineering orrelevant technologies and verify the biological functions thereofwithout requiring creative work.

Fe Domain

The term “Fc domain” or “Fc fragment” refers to the “crystallizablefragment” region of a heavy chain of an immunoglobin. Generally, a Fcdomain can interact with another Fc domain to form a dimer complex. Fcdomain binds to a cell surface receptor (Fc receptor) and/or proteins ofcomplement system, or it can be modified to reduce or enhance suchbinding. Fc domain is derivable from IgG, IgA, IgD, IgM or IgE and hasimmunological functions including Fc receptor dependent procedures suchas opsonization, cell lysis, and mast cells degranulation.

IgG type immunoglobins are among the most abundant proteins in humanblood and have a circulating half-life as long as 21 days. Fusionproteins have been reported to combine the Fc regions of IgG with thedomains of another protein. The prototype fusion protein is ahomodimeric protein liked through cysteine residues in the hinge regionof IgG Fc, resulting in a molecule similar to an IgG molecule withoutthe heavy chain variable and CH1 domains and light chains. The dimernature of fusion proteins comprising the Fc domain may be advantageousin providing higher order interactions (e.g. bivalent or bispecificbinding) with other molecules. Due to the structural homology, Fc fusionproteins exhibit in vivo pharmacokinetic profile comparable to that ofhuman IgG with a similar isotype. To extend the circulating half-life ofIL-15 or IL-15 fusion protein, it is desirable to link the IL-15/IL-15Rαcomplex to the Fc portion of the human heavy chain IgG protein. Theoriginal immunoglobin source of the native Fc is preferably of humanorigin and may be any of the immunoglobins, although IgG1 and IgG2 arepreferred.

In some embodiments, the term “Fc variant” refers to a molecule orsequence that is modified from a native Fc but still comprise a bindingsite for the salvage receptor, FcRn. “Fc domain” comprises a molecule orsequence that is humanized from a non-human native Fc. Furthermore, anative Fc comprises sites that may be removed because they providestructural features or biological activity that are not required for thefusion proteins of the present invention. Thus, in certain embodiments,the term “Fc variant” comprises a molecule or sequence that lacks one ormore native Fc sites or residues. The term “Fc domain” encompassesnative Fc and Fc variant molecules and sequences as defined above,including molecules in monomeric or multimeric form, whether digestedfrom whole antibody or produced by recombinant gene expression or byother means.

RGD Polypeptides

Arg-Gly-Asp (RGD) was found by Pierschbacher and Rouslahti in 1984 in FNas a cell adhesion sequence. They found RGD polypeptide was able toelute integrin α5β1 from affinity column and to adhere to cells whenimmobilized on a matrix material. Then, many glycoproteins (such as LM),collagen, fibrinogen (Fb) in the extracellular matrix were found to havehighly conservative RGD polypeptide, and it was demonstrated to play animportant role in the mediation of the interactions among cell-cell andcell-extracellular matrix proteins.

The biding of RGD polypeptide to a cell is a binding to the integrins onthe cell surface. Integrin was found in 1990s and belongs to Ca²⁺dependent cell surface receptor family. Each integrin comprises 2subunits: α subunit and β subunit. 18 α subunits and 8 β subunits havebeen found so far, constituting 24 types of integrins. Integrin that canrecognize RGD polypeptide and bind thereto include α3β1, α5β1, αllbβ3,α5β1, αvβ1, αvβ3, αvβ5, αvβ6, αvβ8 and so on. RGD polypeptide showsextremely strong affinity and selectivity to αvβ3 integrin. αvβ3integrin was highly overexpressed in various tumor cells and endothelialcells generated by tumor related angiogenesis.

In the invention described herein, the drug was conjugated or fused withRGD polypeptide which directs the selective enrichment of the fusionmolecule to the tumor tissue, resulting in increased local drugconcentration and enhanced tumor killing effects, while limitingsystemic toxicity.

RGD tripeptide is biological inactive and the fourth amino acidadjoining the RGD tripeptide substantially affects its activity. Thefifth amino acid adjoining the RGD tripeptide plays an important role inthe binding specificity. It was demonstrated that addition of residuesat the N terminal of the RGD tripeptide did not interrupt its adherenceto cells. For example, RGD tripeptide and GRGD tetrapeptide do notexhibit significant difference in cell adherence. On the contrary, aminoacid addition at the C terminal would alter its cell adherence. Forinstance, addition of Serine following the Asp residue would enhance thecell adherence activity, while a right-handed residue in replace of aleft-handed residue would damage the cell adherence.

In one embodiment of the present invention, the RGD polypeptide has anamino acid sequence ACDCRGDCFCG, i.e., Ala Cys Asp Cys Arg Gly Asp CysPhe Cys Gly, in which the RGD motif is located at the 5^(th) to 7^(th)amino acids.

In the invention, the term “RGD variant” refers to a polypeptide havingat least one amino acid substitution, deletion or insertion compared tothe RGD polypeptide sequence but still maintaining the integrin receptorbinding function. A person skilled in the art could design one or moreRGD polypeptide variant based on the disclosure of the present inventionand known techniques. Exemplary RGD variants include GRGD, GRGDSPC,GRGDDSY, EPRGDNYR and so on.

Linkers

The fusion proteins of the invention may also include a linker betweencomponents. The linker is normally a short polypeptide comprised of 4 to20 amino acids. The linkers allow effective positioning of eachcomponents to allow functional activity of these domains.

In certain embodiments, in the fusion proteins of the present invention,IL-15 polypeptide is covalently liked to IL-15Rα polypeptide so thatIL-15 and IL-15Rα domains are capable of interacting with each other toform a protein complex. In certain embodiments, the IL-15 and IL-15Rαdomains are effectively positioned to allow interactions with immunecells to initiate or inhibit an immune reaction, or to inhibit orstimulate cell development.

In some embodiments, in the fusion proteins of the invention, IL-15 orIL-15Rα domain is covalently linked to the Fc domains through a linker.The liker sequence should allow effective positioning of the Fc, IL-15or IL-15Rα domains to allow functional activity of each domain. Incertain embodiments, the Fc domains are effectively positioned to allowproper fusion protein complex formation and enhanced in vivo half-lifeof the fusion protein complex.

In some embodiments, in the fusion proteins of the invention, RGDpolypeptide is covalently linked to the Fc domains through a linker. Theliker sequence should allow effective positioning of the RGD and Fcdomains to allow functional activity of each domain. In certainembodiments, the RGD domains are effectively positioned to allow bindingto integrin on tumor cell surface at a high affinity and specificity.

Preferably, the linker sequence comprises from about 2 to 20 aminoacids, more preferably from about 5 to 20 amino acids. The linkersequence is preferably flexible so as not hold the effector molecule ina single undesired conformation. The linker sequence can be used, e.g.,to space the recognition site from the fused molecule. The linkerpreferably predominantly comprises amino acids with small side chains,such as glycine, alanine and serine, to provide for flexibility.Preferably about 80 or 90 percent or greater of the linker sequencecomprises glycine, alanine and serine, particularly glycine and serineresidues. Examples of suitable linker sequence are GGGGS (G₄S), i.e. GlyGly Gly Gly Ser, which is used for example to link the RGD polypeptideto the Fc domain, and the Fc domain to the IL-15Rα polypeptide; andSG₂SG₄SG₃SG₄SLQ, i.e. Ser Gly Gly Ser Gly Gly Gly Gly Ser Gly Gly GlySer Gly Gly Gly Gly Ser Leu Gln, which is used for example to link theIL-15 to IL-15Rα domains. Different linker sequences could be usedincluding any of a number of flexible linker designs that have been usedsuccessfully to join antibody variable regions together. Additionally,suitable size and sequences of linker sequences also can be determinedby conventional computer modeling techniques.

The term “polypeptide’ is meant to refer to any polymer preferablyconsisting essentially of any of the 20 natural amino acids regardlessof its size. Although the term “protein” is often used in reference torelatively large proteins, and “peptide” is often used in reference tosmall polypeptides, use of these terms in the field often overlaps. Theterm “polypeptide variant” refers to an amino acid sequence has one ormore amino acid substitution, deletion or insertion compared to thepolypeptide sequence but still maintaining the biological function ofthe polypeptide.

The term “vector” is a nucleic acid molecule that is able to replicateautonomously in a host cell and can accept foreign DNA. A vector carriesits own origin of replication, one or more unique recognition sites forrestriction endonucleases which can be used for the insertion of foreignDNA, and usually selectable markers such as genes coding for antibioticresistance, and often recognition sequences (e.g. promoter) for theexpression of the inserted DNA. Common vectors include plasmid vectorsand phage vectors.

Materials and Methods

-   Antibodies: Recombinant human IL-2 (AF-200-02) and    granulocyte-macrophage colony-stimulating factor (300-03) were    purchased from Peprotech. Recombinant human IL-15 (247-IL-105) was    purchased from R&D Systems. Anti-MsCD3e (145-2c11)-PerCP,    anti-MsCD8a(53-6.7)-FITC, anti-MsNK1.1(PK136)-FITC,    anti-MsCD44(IM7)-PE and anti-MsCD122(TM-Btal)-PE were purchased from    BD PharMingen. Anti-human CD51/61 (αvβ3 integrin) purified mAb was    purchased from eBioscience. Goat anti-human IgG(H⁺L)-AlexaFluor 488,    goat anti-mouse IgG(H⁺L)-AlexaFluor 488 and goat anti-Mouse    IgG(H⁺L)-AlexaFluor 647 were purchased from Invitrogen.-   Cell lines and animals: SKOV-3, CTLL-2, and Mo7e cells were    purchased from the Shanghai Cell Bank. HUVEC cells were kindly    gifted by Dr. Gao Huile from Sichuan University. CTLL-2 cells were    cultured in RPMI 1640 supplemented with 20% fetal bovine serum    (FBS), 30 ng/ml IL-2, and 1% non-essential amino acids. Mo7e cells    were cultured in RPMI 1640 supplemented with 10% fetal bovine serum    (FBS), 10 ng/ml GM-CSF, and 1% non-essential amino acids. SKOV-3 and    HUVEC cells were cultured in DMEM supplemented with 10% FBS. PBMC    were isolated from the buffy coat of healthy donors using    Ficoll-Paque plus (GE health) and cultured in RPMI-1640 supplemented    with 10% FBS. C57bl/6 mice were purchased from the Animal Experiment    Facility of Sun Yat-sen University. Human blood collection, animal    care and animal experiments were approved by Sun Yat-sen University.

Expression and Purification of the Fusion Protein

Generally, the fusion proteins of the invention could be prepared by theprocedures disclosed herein or other DNA recombinant techniques in theart, for example, PCR, plasmid DNA extraction, DNA digestion byrestriction endonucleases, DNA ligation, mRNA isolation, introduction ofDNA into suitable cells, transformation and transfection of host cells,culture of host cells and so on. Additionally, the fusion proteins canbe isolated and purified by suitable agents and known methods, includingelectrophoresis, centrifugation, chromatography and etc.

To generate the recombinant protein PFC-1, the fusion gene (PFC-1) wascloned into the pcDNA3.1 (+) vector with a mouse kappa chain signalpeptide. The plasmid was transiently transfected to 293 cells. Onehundred ml of media were collected after 3 d of culture. The PFC-1protein was purified with a Protein-A-agarose affinity purificationprotocol.

The fusion protein PFC-1 is shown in FIG. 1, which comprises thefollowing 3 parts: (1) IL-15/IL15Rα complex, (2) an Fc domain, and (3) aRGD peptide. The parts are linked by a linker GGGGS, and a His-tag isattached to the C terminal of the fusion protein (FIG. 1). The DNAsequence was cloned into the pCDNA3.1(+) vector and then transientlytransfected into HEK293 cells to express.

A single band of approximately 60 kDa was observed under reducingconditions (FIG. 2). Under non-reducing conditions, the majority of theprotein is approximately 120 kDa with a minor product at approximately60 kDa. The results suggested that a homogenous PFC-1 fusion protein wasobtained by mammalian expression and affinity purification, and themajority of the PFC-1 protein was in a dimeric form.

Cytokine-Dependent Cell Proliferation Assay

To measure cytokine-dependent cell proliferation, CTLL-2 and Mo7e cellswere harvested in their logarithmic growth phase, washed twice with PBSand incubated for 4 h in assay medium (RPMI 1640 supplemented with 10%FBS and 1% NEAA) for cytokine starvation at 37° C. and 5% CO2. Duringthe incubation, IL-15 and PFC-1 were diluted to an initial concentrationof 10 nM in the assay medium, followed by serial dilutions. After a 4-hincubation, cells were collected and a cell suspension (2×10⁴cells/well) was seeded immediately into corresponding wells andincubated at 37° C. and 5% CO2 for 48 or 72 h with CTLL-2 or Mo7e cells,respectively. After a 48-h or 72-h incubation period, CCK-8 assay(Dojindo) was performed to measure the amount of live cells.

CTLL-2 is a murine cytotoxic T lymphocytic cell line with positiveexpression of both the IL-15Rα chain and the IL-15βγ complex, while Mo7eis a human megakaryocytic leukemic cell line that only expresses theIL-15βγ complex. Proliferation of both cell lines can be induced by thepresence of IL-15. Similar to IL-15, PFC-1 can stimulate theproliferation of both Mo7e and CTLL-2 cell lines (FIGS. 3 and 4),demonstrating the IL-15 cytokine activity of PFC-1

When the molar concentration of PFC-1 was calculated as a monomer inMo7e cells, rhIL-15 showed a slightly higher cytokine activity thanPFC-1(FIG. 3). However, no difference was observed at 10 nM. PFC-1worked better than rhIL-15 in CTLL-2 cells, with approximately 2- to4-fold stronger activity (FIG. 4).

CFSE Labeling of PBMC and Proliferation Assay

To measure PBMC proliferation, PBMCs were freshly prepared by Ficollcentrifugation, adjusted to 2×10⁶ cells/ml, and then stained with 5 μMCFSE (eBioscience) according to the manufacturer's instructions. StainedPBMCs (5×10⁵ cell/ml) were incubated with 1 nM or 10 nM of rhIL-15 andPFC-1 for 6 d PBMC proliferation was assessed by flow cytometry using aCytomic FC500 (Beckman Coulter) and analyzed using the Kaluza software(Beckman Coulter).

To measure the activity of PFC-1 on primary immune cells, PBMCs(Peripheral blood mononuclear cells) were prepared, stained with CFSEand incubated with rhIL-15 or PFC-1 for 6 d. Both rhIL-15 and PFC-1, ateither 1 or 10 nM, significantly stimulated the proliferation of PBMCscompared to the control group (FIG. 5). However, different from the Moleand CTLL-2 cytokine-dependent proliferation assay, PFC-1 exhibited anapproximately 10-fold stronger potency than rhIL-15 in PBMCproliferation, while 10 nM rhIL-15 led to a proliferation rate of 20.71%and 1 nM PFC-1 resulted in a mean value of 22.05% (FIG. 5). The enhancedactivity could be due to the higher activity of IL-15/IL-15Rα thanIL-15, the extended half-life by the Fc fragment, or both.

Colocalization of PFC-1 and Integrin

To confirm that PFC-1 can indeed bind to tumor cells or tumorendothelial cells through cell surface integrins, HUVEC endothelialcells or SKOV3 ovarian cancer cells were used to check PFC-1 binding asboth of these cell lines have high expression of αvβ₃ integrins. Flowcytometry analysis suggested that both HUVEC (FIG. 6) and SKOV3 cells(FIG. 7) are indeed αvβ₃-integrin positive. The colon cancer cell lineLS174T is αvβ₃-integrin negative (FIG. 8). Therefore, these three celllines were used to verify the colocalization of the PFC-1 and integrin.

Cells were trypsinized, adjusted to 4×10⁵ cells/ml and incubated incomplete DMEM medium for 2 hr at 37° C. Cells were then washed with PBSand aliquoted to a concentration of 2×10⁵ cell/500 μl. Then, the cellswere stained with 2 μg of anti-human CD51/61 (αvβ₃ integrin) or PFC-1,followed by incubation with a fluorophore-conjugated secondary antibodybefore being subjected to flow cytometry analysis.

When coincubated with HUVEC or SKOV3 cells, PFC-1 can bind to HUVEC(FIG. 6) and SKOV3 (FIG. 7) cells, though weaker than the anti-αvβ₃integrin mAb. However, no binding was observed for the colon cancer cellline LS174T, regardless of PFC-1 or the anti-αvβ₃ integrin mAb (FIG. 8).Flow cytometry shows that PFC-1 could specifically bind to the HUVEC orSKOV3 cell surfaces.

Confocal Microscopy

SKOV3 and HUVEC cells were cultured on 30-mm glass-bottom dishes (InVitro Scientific) to 70% confluence. The cells were then washed withcold PBS and fixed with 4% paraformaldehyde. The fixed cells wereincubated with 2 μg of purified anti-human CD51/61 (αvβ₃ integrin) orPFC-1 at room temperature for 1 hr, followed a second incubation at roomtemperature for 1 hour with either goat anti-mouse IgG-AlexaFluor 647 orgoat anti-human IgG-AlexaFluor 488, respectively. The nuclei werecounterstained with DAPI. Zeiss LSM710 confocal microscopy was used toobserve the cells.

Imaging analysis showed that PFC-1 co-localized with the anti-αvβ₃integrin mAb on both HUVEC and SKOV3 cells (FIG. 9), indicating PFC-1and the anti-αVβ3 integrin mAb bind to the same cell surface protein(i.e., αVβ3-integrin in this case), although via different proteinepitopes.

The results of flow cytometry and confocal microscopy suggested thatPFC-1 specifically binded to αvβ₃-integrin via RGD motif and thereforeachieved specific targeting to αvβ₃-integrin expressing tumor cells invitro.

PFC-1 has High In Vivo Anti-Tumor Efficacy

For the anti-tumor studies, 4-6-week old female C57bl/6 mice wereinjected with 5×10⁵ B16F10 mouse melanoma tumor cells at the rightflank. Ten to 12 d later, once tumors reached 5-8 mm in diameter (day0). When the tumors reached 50-100 mm³, the mice were administered withdifferent dosages of PFC-1 or negative control (PBS). The tumor size wasmeasured at scheduled dates. Results showed that 5 μg PFC-1 treatmentwas able to restrain tumor growth by 70%, while the 20 μg PFC-1treatment was able to completely (100%) abrogate tumor growth (FIG. 10).

In a similar experiment in mouse model, 4-6-week old female C57BL/6 micewere inoculated intravenously with 5×10⁵ B16F10 cells. After large tumorburdens were established, 10 μg PFC-1 was injected intravenously for 2consecutive days. Potent in vivo tumor growth blockage was observed(FIG. 11). On day 5, the tumor volume had shrunk by 25% after PFC-1treatment (FIG. 11) compared to the initial tumor volume. With anadditional PFC-1 treatment on day 6, tumor volumes decreased to 54% ofthe initial tumor volume.

Phenotypic Characterization by Flow Cytometry

Peripheral blood was collected from the orbital vein. Spleens wereremoved and splenocytes were processed into single cell suspensions andfiltered through a 70-μm nylon mesh (BD). Tumor tissues were alsoremoved and gently disrupted with forceps followed by enzymaticdigestion with 0.2 mg/ml collagenase IV (Sigma) and 0.1 mg/ml DNase I(Sigma) in RPMI-1640 at 37° C. for 15 min. The released cells werecollected and the remaining tumor tissue was subjected to furtherprocessing by incubation in fresh digestion medium for an additional 25min at 37° C. Single-cell suspensions were filtered with a 70-um nylonmesh. Cell samples from the blood, spleen and tumor tissues were thenstained with the corresponding antibodies at room temperature for 30min, protected from light. Samples were then washed with PBS twice andadjusted to an appropriate volume for flow cytometry analysis.

Flow cytometry was performed to analyze lymphocytes isolated from theperipheral blood, spleen and tumors of mice treated with PFC-1 or thevehicle groups. A steep increase in the number of CD8⁺T cells wasobserved in the peripheral blood, spleen and tumors of mice treated withPFC-1 (FIG. 12). A significant increase in CD44, a T cell activationmarker, on CD8⁺T cells was also observed in peripheral blood,splenocytes, and tumor-infiltrating lymphocytes, suggesting that PFC-1not only increased the population but also activated CD8⁺T cells. PFC-1was also able to mobilize more NK cells in tumors (FIG. 13). All of thedata suggest that PFC-1stimulate immune cells to kill tumor cells.

PFC-1 Blocks B16 Tumor Metastasis in Mouse

5×10⁵ B16F10 melanoma cells was injected via tail vein to inoculate4-6-week female C57BL/6 mice. Lung metastasis developed rapidly in thesemice. On day 2, a single dosage of 10 μg PFC-1 or equal volume ofvehicle was administered. On day 21, the mice were sacrificed and thelungs were removed, washed thoroughly with PBS and fixed in 10%formaldehyde. The metastasis nodes were counted under binocularmicroscope (Leica M125).

Results showed that a single intraperitoneal administration of 10 μgPFC-1 effectively reduced the average lung metastasis node number from61, the average in the vehicle group, to 11.4 in the treated group, adecrease of more than 80% (FIG. 15), suggesting that PFC-1 caneffectively block tumor metastasis.

The present technology illustratively described herein may suitably bepracticed in the absence of any element or elements, limitation orlimitations, not specifically disclosed herein. It should be understoodthat the materials, methods, and examples provided here arerepresentative of preferred aspects, are exemplary, and are not intendedas limitations on the scope of the present technology.

1. A tumor-targeting fusion protein, comprising at least (i) a IL-15peptide or a variant or functional fragment thereof, (ii) a IL-15Rαpolypeptide or a variant or functional fragment thereof, (iii) a Fcdomain or a variant or functional fragment thereof, and (iv) a RGDpolypeptide or a variant thereof.
 2. The tumor-targeting fusion proteinof claim 1, wherein the fusion protein is configured as RGDpolypeptide-Fc domain-IL-15 polypeptide-IL-15Rα polypeptide.
 3. Thetumor-targeting fusion protein of claim 1, wherein the Fc domain has anamino acid sequence SEQ ID NO:1 or an equivalent thereof that hassubstitution, deletion or addition of one or more amino acids withrespect to the sequence of SEQ ID NO:1 but retains the function of theFc domain.
 4. The tumor-targeting fusion protein of claim 1, wherein theIL-15Rα has an amino acid sequence SEQ ID NO:2 or an equivalent thereofthat has substitution, deletion or addition of one or more amino acidswith respect to the sequence of SEQ ID NO:2 but retains the function ofthe IL-15Rα.
 5. The tumor-targeting fusion protein of claim 1, whereinthe IL-15 has an amino acid sequence SEQ ID NO:3 or an equivalentthereof that has substitution, deletion or addition of one or more aminoacids with respect to the sequence of SEQ ID NO:3 but retains thefunction of the IL-15.
 6. The tumor-targeting fusion protein of claim 1,wherein the RGD polypeptide has an amino acid sequence SEQ ID NO:4 or anequivalent thereof that has substitution, deletion or addition of one ormore amino acids with respect to the sequence of SEQ ID NO:4 but retainsthe function of the RGD polypeptide.
 7. The tumor-targeting fusionprotein of claim 1, wherein the tumor-targeting fusion protein has anamino acid sequence selected from a group consisting of: (a) an aminoacid sequence of SEQ ID NO: 5; (b) an amino acid sequence encoded by anucleic acid sequence of SEQ ID NO:6; (c) an amino acid sequence encodedby a degenerate sequence of the nucleic acid sequence of SEQ ID NO:6;and (d) an equivalent amino acid sequence of SEQ ID NO: 5 that hassubstitution, deletion or addition of one or more amino acids butretains the function of the fusion protein.
 8. The tumor-targetingfusion protein of claim 1, wherein the tumor is an integrin positivetumor.
 9. The tumor-targeting fusion protein of claim 8, wherein thetumor is a αVβ3 integrin positive tumor.
 10. The tumor-targeting fusionprotein of claim 1, wherein the tumor is a progressive tumor, anadvanced tumor, a tumor with a high burden/load, or a metastatic tumor.11. A pharmaceutical composition comprising the tumor-targeting fusionprotein of claim 1, and a pharmaceutically acceptable excipient.
 12. Apharmaceutical composition comprising the tumor-targeting fusion proteinof claim 1, and a further anti-tumor agent.
 13. A nucleic acid sequenceencoding the tumor-targeting fusion protein of claim
 1. 14. Anexpression vector comprising the nucleic acid sequence of claim
 13. 15.A host transformed or transfected by the expression vector of claim 14.16. A kit comprising the nucleic acid sequence of claim 13.